GPS signals play an ever increasing role in our commercial, civil and military enterprises, providing everything from cell tower synchronization to delivery truck tracking and unmanned vehicles navigation. This reliance on GPS makes the growing frequency, power and sophistication of interference affecting its reception a substantial threat to our economy, as well as our homeland and national security.
Disruptive jamming and spoofing, a particularly insidious form of jamming, apparently were used by Iran to capture a US drone. Spoofing signals are GPS mimics that are broadcast intentionally in the GPS band to mask the actual signals and suborn the drone autopilot to lead the vehicle astray. Furthermore, more powerful signals from easily fabricated transmitters can blind a GPS receiver, potentially causing airline crashes as well as disrupting our national infrastructure.
Technology proposed to combat interference affecting GPS or other radio frequency (RF) receivers can be classed generally as digital signal processing, analog filtering, or steered antennas. Digital signal processing methods are sophisticated and diverse but depend on signals with an adequate signal to noise ratio (SNR) and preferably free of distortion before they are digitized. These methods can also require long signal records and substantial computation. Analog bandpass filters improve signal to noise ratio by rejecting frequencies outside a desirable frequency range. While this reduces the risk of distortion, it is ineffective against interference occurring at passband frequencies. Array steering is used to reduce receiver sensitivity in the direction of a jamming source but has several disadvantages: the hardware imposes a substantial burden in terms of size, weight, power, cost, and computational complexity; and array steering equally reduces sensitivity to desirably received signals from the direction of the null and other bearings represented by null side lobes. As a result, null steering is largely restricted to ground stations or large vehicles with the payload capacity and energy to provide the required hardware and power.
Conventional systems for defeating interference rely on temporal or spatial diversity. Temporal diversity excises from an antenna signal periods of time when interference is present to avoid its degrading a signal of interest (SI). Spatial diversity relies on differences in direction of propagation between interference and SI, acting to reduce sensitivity of the receiving system to signals from the direction of the interference. Sensitivity can be reduced by mechanically or electrically steering a null in the direction of the interference or by using directional antennas oriented in the direction of the interference and of SI. In this, defeat of interference without degrading SI requires a large number of array antenna elements, computationally intensive calculation of weightings applied to array element signals, and/or steered directional antennas. Such spatial diversity technology, however, also degrades SI propagating from a direction proximate that of the interference. And, the use of fewer array elements and/or less directional antennas broadens the null, resulting in cancellation of SI over wide ranges of bearing and reducing overall system performance. In light of this, technology that can defeat interference at any bearing including those proximate in bearing to SI, and doing so without large numbers of array elements or use of directional antennas, is clearly desirable.
In light of the above, we propose GPS receivers comprising selective cancellation of spoofing and other types of in-band interference using omnidirectional receiving antennas providing greater angular operating range and cancellation bandwidth at reduced cost and complexity.